Stereoscopic imaging is the process of visually combining at least two images of a scene, taken from slightly different viewpoints, to produce the illusion of three-dimensional (3D) depth. This technique relies on the fact that human eyes are spaced some distance apart and do not, therefore, view exactly the same scene. By providing each eye with an image from a different perspective, the viewer's eyes are tricked into perceiving depth. Typically, where two distinct perspectives are provided, the component images are referred to as the “left” and “right” images, also known as a reference image and complementary image, respectively. However, those skilled in the art will recognize that more than two viewpoints may be combined to form a stereoscopic image.
The transition from film to digital cinema has created new opportunities for cinema such as in 3D and real time transmission. For 3D content, it is expected that stereoscopic content will dominate digital cinema for the foreseeable future. Stereoscopic digital cinema involves projecting two images using a single projector. There are currently three techniques for projecting stereo content in digital cinema as described below:                Use active glasses with no optical modifications: In this technique, a projector alternates left and right images on a regular screen. The user or viewer wears active glasses that is synched to the content using an IR controller.        Use passive glasses with circular polarizer: This is currently the most widely deployed system. The projector also alternates left and right images on a sliver screen. A polarization unit is placed in front of the projector. The user or viewer is required to wear special passive glasses.        Dolby 3D technique: In this technique, colors are divided into six bands, three for each eye. The projector alternates left and right images and the user or viewer wears a special color filtering glasses.As described above all techniques require glasses to view the content. Without the glasses, the 3D content would look distorted.        
In the cinema, it is common that a commercial advertisement, e.g., a movie preview, is typically two-dimensional (2D). Then, there is a need to let the viewers know when to wear the special glasses to realize the 3D effect. To let the viewer know when to wear the glasses, the cinema may use a curtain moving up and down, or they have to make an announcement. This becomes difficult since there is potentially many transitions from 3D to 2D and vice versa during the show or film. Therefore, a need exists for techniques to provide a transition message to a viewer of a stereoscopic film that does not provide discomfort to the viewer.
Furthermore, piracy concerns in connection with the theatrical exhibition of motion picture films are well known. Once a film distributor distributes prints of a motion picture film to exhibitors for theatrical exhibition, often a certain degree of control over the product is lost. For example, in the regular course of exhibiting the film in a theater, whether analog or digital, a customer in the theater may surreptitiously record the film using, e.g., a hand held camcorder during the exhibition. As a result, an illicit copy of relatively high quality can be made. Such illicit “pirate” copies of a movie can now be distributed over the Internet or by using hard copies (video CDs or DVDs), and this reduces the economic value of the legitimate film distribution. With the advent of the Internet and affordable higher quality recording devices, this problem has become increasingly severe in recent years. Thus, a further need exists for techniques for providing warning messages against illegal copying of films which are not visible to a legitimate viewer of the film.